This invention relates to electronic devices which are encapsulated by a polymeric encapsulant, and more particularly, to such devices which are encapsulated by a silicone gel.
Silicone resins have been used for a number of purposes because of their relative thermal stability, dielectric properties, chemical stability and resistance to atmospheric deterioration. U.S. Pat. No. 4,564,562 of C. P. Wong, assigned to a subsidiary company of the present assignee, is one example of the prior art showing the use of silicone resins as an encapsulant for electronic devices particularly, hybrid integrated circuits. A hybrid integrated circuit is one in which a semiconductor chip, usually a silicon chip, is connected to a bonding pad region of a circuit pattern on a ceramic substrate. The Wong patent is directed to the use of certain additives in certain resins that improve their adhesion to gold and tantalum nitride surfaces, which are sometimes the metals used as the circuit patterns.
Wire bonding is a commonly used technique for interconnecting integrated circuit chips with substrates which involves forming a delicate wire that extends in an arc from the chip to the substrate. Silicone resins such as described in the Wong patent and various expoxies have been found to be sometimes undependable when used with wire bonded chips because the inevitable thermal stresses during curing tend to break the delicate interconnection wires. Silicone gels, which are platinum catalyzed silicone resins, either polydimenthylsiloxane or polymethylphenylsiloxane, having vinyl and hydride reactive functional components, are sufficiently elastic to encase the interconnection wires without breaking them, but after cure they retain a relatively soft rubber-like or jelly-like constituency. While this is adequate for many purposes, it will not protect delicate silicon chips from the effects of relatively rough handling. In particular, it tends to rip or tear if impacted, say, by a concern of a rigid object. Moreover, it is not sufficiently resistant to solvents such as Freon (a trademark of the Dupont Co.) which are used to clean hybrid integrated circuit substrates. For a more complete discussion of silicone gel encapsulations for electronic devices see "Application of Polymer in Encapsulation of Electronic Parts," by C.P. Wong, "Advances in Polymer Science," Vol. 84, Springer-Verlag, Berlin, Heidelberg, 1988, p. 64 at p. 78.
Alternative approaches to device encapsulation include packages made of hard rigid material such as plastic or ceramic that do not physically contact the chip or its leads. While such packages can withstand the rigors of rough handling, they do not encase the elements of an electronic device as a silicone resin does, and may therefore offer less protection from shocks and the like. Many materials such as plastic cannot encase a chip because of inadequate electrical properties; other materials such as epoxy cannot be used in this manner because thermal stresses during fabrication would break the delicate bonding wires. Thus, it would be beneficial to provide a encapsulant that is capable of flowing about an electronic device to encase it, has appropriate adhesive, electrical, thermal, solvent resistant and shock resistant properties, and still has a sufficiently hard outer surface to resist the effects of rough handling.